Electrochemical cells in which a chemical reaction is forced by adding electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
A typical electrochemical cell will have a positively charged anode and a negatively charged cathode. The anode and cathode are typically submerged in a liquid electrolytic solution which may be comprised of water and certain salts, acids or base materials. Generally speaking, the anode and cathode are made of substrate materials such as titanium, graphite, or the likes coated with a catalyst such as lead dioxide or other known materials. The selection of a substrate and catalyst is determined by the particular electrode reactions which are to be optimized in a given situation.
The cathode and anode are positioned within the electrolytic cell with electrical leads leading to the exterior. The cell may be provided with appropriate plumbing and external structures to permit circulation of the electrolyte to a separate heat exchanger. Suitable inlet and outlet passages may also be provided in the cell head space to permit the withdrawal of the gases evolved from the cathode (if gases are to be evolved) and from the anode.
In order to maintain or reduce the temperature of the cell electrodes, heat exchange passages may be provided within the electrode structures. These coolant passages are connected to external sources of coolant liquid which can be circulated through the electrodes during the electrolysis process in order to maintain or reduce their temperatures.
In order to drive the electrolysis reactions it is necessary to apply electric power to the cell electrodes. The electrodes are connected through the electrical leads to an external source of electric power with the polarity being selected to induce the electrolyte anion flow to the anode and the cation flow to the cathode.
Layer-by-Layer (LBL) Technique
Organic thin films continue to attract great interest in the materials science community due to their ease of processing, ease of functionalization, light weight and flexibility. Significant progress has been achieved in the past 10-20 years, presenting the possibility of molecular-level control in molecular and macromolecular composite films. The ionic, layer-by-layer assembly technique, introduced by Decher in 1991, is among the most exciting recent developments in this area. Makromol. Chem., Macromol. Symp. 1991, 46, 321; Ber. Bunsenges. Phys. Chem. 1991, 95, 1430; Thin Solid Films 1992, 210/211, 831. This approach, which utilizes electrostatic interactions between oppositely charged polyion species to create alternating layers of sequentially adsorbed polyions, provides a simple and elegant means of depositing layer-by-layer sub-nanometer-thick polymer films onto a surface using aqueous solutions. Crystallography Reports 1994, 39, 628; Macromol. 1995, 28, 7107; Langmuir 1997, 13, 2171. This layer-by-layer deposition process provides a means to create polycation-polyanion polyelectrolyte multilayers one molecular layer at a time, thereby allowing an unprecedented level of control over the composition and surface functionality of these interesting materials. Typically, alternate layers of positively and negatively charged polymers are sequentially adsorbed onto a substrate from dilute solution to build up interpenetrated multilayer structures. Most studies have focused on polyelectrolytes in their fully charged state, such as strong polyelectrolyte poly(styrene sulfonate) (SPS). However, we have discovered unique properties when at least one alternating layer in the polyelectrolyte multilayer is a weak polyelectrolyte where the charge density along the chain can be readily controlled by adjusting the pH values of the polyelectrolyte solution. Thin Solid Films 1992, 210, 831.
More recently, applications have been extended to electroluminescent LEDs, conducting polymer composites, and as the assembly of proteins and metal-nanoparticle systems. Adv. Mater. 1995, 7, 395; Adv. Mat. 1998, 10, 1452; Thin Solid Films 1994, 244, 985; Thin Solid Films 1994, 244, 806; J. Am. Chem. Soc. 1995, 117, 6117. The electrostatic LBL technique has been extended to include many charged systems other than polymers and even other complexation mechanisms, such as hydrogen bonding. Chem. Lett. 1997, 125; Macromol. 1997, 30, 2717.
Solid Polymer Electrolytes (SPEs)
As mentioned above, all electrochemical systems consist of electrodes separated by an electrolyte for ion conduction and a load for electronic conduction, as electricity can be generated or fed into the system. Early electrochemistry relied exclusively on liquid electrolytes, but recent applications are more demanding. Solid polymer electrolytes (SPEs) have replaced liquid electrolytes in many high-performance applications, such as batteries, fuel cells, sensors, and electrochromic devices. Compared to liquid electrolytes, SPEs feature easier processing, enhanced chemical compatibility, and better mechanical properties with only a modest decrease in conductivity.
A major advantage gained from forming SPEs by the LBL technique is the introduction of a large number of variables that modify the electrolyte or the electrodes depending on the user's application. Other advantages include the utilization of cheap nontoxic polyelectrolyte materials, an economic and simple fabrication process, and miniaturization of the electrochemical components. For example, a composite membrane made by LBL deposition of a poly(+)/poly(−) couple on a porous framework is more than ten times cheaper than any common commercial proton-exchange-membrane (PEM), yet it can deliver more than half the power. In addition, a stainless steel composite electrode made by LBL deposition of a colloid of platinum/carbon catalyst with a poly(−)/poly(+) stabilizers acted similar to a pure platinum electrode by furnishing the same open-circuit potential yet it is a thousand times cheaper and, unlike solid platinum, allows the conduction of ions.
Fabrication of Fuel Cells Via LBL
A fuel cell is a type of electrical energy generating device. There are several types of fuel cells such as acid fuel cells, molten carbonate fuel cells, solid polymer electrolyte fuel cells and solid oxide fuel cells. A fuel cell is an apparatus for continually producing electric current by electrochemical reaction of a fuel with an oxidizing agent. More specifically, a fuel cell is a galvanic energy conversion device that chemically converts a fuel such as hydrogen or a hydrocarbon and an oxidant that catalytically react at electrodes to produce a DC electrical output. In one type of fuel cell, the cathode material defines passageways for the oxidant and the anode material defines passageways for fuel. An electrolyte separates the cathode material from the anode material. The fuel and oxidant, typically as gases, are continuously passed through the cell passageways for reaction. The essential difference between a fuel cell and a battery is that there is a continuous supply of fuel and oxidant from outside the fuel cell. Fuel cells produce voltage outputs that are less than ideal and decrease with increasing load (current density). Such decreased output is in part due to the ohmic losses within the fuel cell, including electronic impedances through the electrodes, contacts and current collectors. A need therefore exists for fuel cells which have reduced ohmic losses.
Recently, industrial nations have revived the usage of alternative energy sources to address their energy problems. At the forefront of alternative energy technologies are fuel cells which consume hydrogen or methanol, rather than crude oil, to generate electricity. Larminie, J.; Dicks, A. Wiley, New York 2000. Although fuel cell technologies are relatively well known, there is a strong need for more portable, lightweight and low-cost fuel cell devices for portable devices, micropower applications, and new applications requiring embedded power in textiles, paper, plastics and other thin film geometries. Using the layer-by-layer (LBL) self-assembly a new generation of fuel cells can be envisioned. Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831-835; Arys, X.; Jonas, A. M.; Laschewsky, A.; R., L. 2000, 505-564. Micro fuel cells assembled using the LBL technology, are fundamentally different from those described in the literature. Most authors use expensive lithographic and sputtering techniques to fabricate a large number of microelectrodes on a flat substrate and use conventional PEMs as separators. However, micropatterned LBL fuel cells would provide access to low cost, readily available, and easily mass-produced micropower devices analogous to, but much cheaper than, the traditional microelectronic processes. Such systems might include the use of an ultrathin perm-selective membrane on a porous, ionically transmissive support. A major advantage of PEMs over classical membranes is that extremely thin films can effectively reduce the flow of specific gases, while maintaining a high flux of others. Krasemann, L.; Tieke, B. Journal of Membrane Science 1998, 150, 23-30; Krasemann, L.; Tieke, B. Material Science and Engineering 1999, 819, 513-519; Krasemann, L.; Tieke, B. Mat. Sc. Eng. C-Bio S89 1999, 513-518; Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287; Krasemann, L.; Tieke, B. Chem. Eng. Tech. 2000, 2, 211-213. With a typical thickness per layer pair of about 1.0 to about 100 nm, it is possible to engineer a broad range of systems which will act as effective components in proton exchange membranes. Krasemann, L.; Tieke, B. Journal of Membrane Science 1998, 150, 23-30; Levasalmi, J. M.; McCarthy, T. J. Macromolecules 1997, 30, 1752.
The core of a fuel cell is the membrane-electrode assembly (MEA). The MEA of a fuel cell is usually fabricated by sandwiching a proton-exchange membrane (PEM) between two gas diffusion C/Pt electrodes. Larminie, J.; Dicks, A. Wiley, New York 2000; Gottesfield, S.; Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301. The most commonly used PEMs are the perfluorosulfonated membranes (e.g., Nafion®) which are comprised of a PTFE crosslinked hydrophobic backbone impregnated with hydrophilic sulfonic acid sites needed for proton mobility. Larminie, J.; Dicks, A. Wiley, New York 2000; Gottesfield, S.; Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301; Mehta, V.; Cooper, J. S. J. Power Sources 2003, 114, 32-53. Other types of membranes used as PEMs are the hydrocarbon polymer, non-fluorinated, and polymer-inorganic composite membranes that, in general, are less expensive and recyclable. Glipa, X.; Hograth, M. Dept. of Trade and Industry (UK) homepage 2001; Panero, S.; Ciuffa, F.; D'Epifano, A.; Scrsati, B. Electrochim. Acta 2003, 48, 2009-2014; Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463-1502. Some polymers such as the polyphosphazenes, the polybenzimidazoles (PBI) and zirconia-polymer gels exhibit an equal or better performance than the conventional perfluorinated membranes, especially for water retention at high temperature. Qunhui, G.; Pintauro, P. N.; Tang, H.; O'Connor, S. J. Mem. Sci. 1999, 154, 175-181; Glipa, X.; Bonnet, B.; Mula, B.; Jones, D. J.; Rozier, J. J. Mater. Chem. 1999, 9, 3045-3049; Alberti, G.; Casciola, M. Solid State Ionics 2003, 145, 3-16. However, it should be noted that the polyphosphazenes and the zirconia-polymer gels are not commercially available and the PBIs are relatively expensive. An emerging membrane technology based on the layer-by-layer deposition of polyelectrolytes multilayer films on solid substrates or detachable films might be harnessed to perform like a classical PEM. Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831-835; Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368-5369; Vazquez, E.; Dewitt, D. M.; Lynn, D. M.; Hammond, P. T. J. Am. Chem. Soc. 2003, 125, 11452; Arys, X.; Jonas, A. M.; Laschewsky, A.; R., L. 2000, 505-564.
Because the LBL films can be tailored to deposit any polyelectrolyte (PE) couple to any desired thickness, ranging from a few angstroms to a few microns, they are much less expensive technology than conventional membranes. Ion permeability and ion conductivity in LBL films have been extensively studied and characterized. Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287; Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184-1192; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 4627-4636; Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978; DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15, 1165-1173; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001. The diffusion coefficient of ions of conventional polymer multilayers is a few orders of magnitude lower than the classical ion exchanger membranes hence their proton conduction is lower. However, a range of multilayer systems which incorporate hydrophilic polymers using electrostatic and hydrogen bonding mechanisms, and have shown increases in ionic conductivity of 3 or 4 orders of magnitude. DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15, 1165-1173; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001; Tokuhisa, H.; Hammond, P. T. Adv. Funct. Mater. 2003, 13, 831-839. These differences are further enhanced by the fact that ultra thin films can be formed using the LBL technique, making the final conductance closer to that required for power applications. One can tune the thickness and permeability, as well as the composition, of these films through choice of polyelectrolytes and adsorption conditions. For example, using strong polyelectrolytes with hydrocarbon backbones yields LBL films that tend to be either strongly or moderately hydrophobic, thus discouraging proton exchange. On the other hand, LBL films assembled using weak electrostatic and secondary interactions (i.e. long-range hydrogen bonding or dipole-dipole), particularly those with hydrophilic backbones, support proton-exchange.
The advantages gained using polyelectrolytes to synthesize the LBL PEM membrane should apply to the synthesis of LBL electrodes. On top of fast ion conduction LBL electrodes demand high electronic conduction, strongly hydrophobic to expel water, stable to chemical and mechanical degradation, assessable to control loading of catalysts, intimately adhere to the PEM and the GDL to ensure proper passage of the ions, capable of producing open-circuit-potentials similar to a pure metal. Larminie, J.; Dicks, A. Wiley, New York 2000; Gottesfield, S.; Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301; Glipa, X.; Hograth, M. Dept. of Trade and Industry (UK) homepage 2001. Conducting polymers were successfully used to assemble LBL electronically conducting films. Rubner, M. F.; Stockton, W. B. Macromolecules 1997, 30, 2717-2725; Rubner, M. F.; Fou, A. C. Macromolecules 1995, 21, 7115.; Rubner, M. F.; Cheung, J. H.; Fou, A. F. Thin Solid Films 1994, 244, 985; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001; DeLongchamp, D. M. PhD Thesis, Massachusetts Institute of Technology, MA 2003. Unfortunately, LBL conducting polymer films are weak ionic conductors, not stable and degrade in a sever electrochemical environment. A more resilient combination is a polyelectrolyte-colloid such that the colloid is electronically conducting and ready to assemble. Many colloids can assemble with polyelectrolytes but the LBL films are not conducting. Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065.; Mallouk, T. E.; Feldheim, D. L.; Crabar, K. C.; Natan, M. J. J. Am. Chem. Soc. 1996, 1181, 7640-7641.; Grabar, K. C.; Natan, M. J.; Freeman, R. G.; Hommer, M. B. Anal. Chem. 1995, 67, 735-743.; Hammond, P. T.; Rubner, M. F.; Zheng, H. P.; Lee, I. Adv. Mater. 2002, 14, 569-572. Only one original approach used exfoliated graphite oxide that is not conducting to make LBL films because graphite cannot be dispersed in water and it forms micrometer-sized irregular aggregates in organic solvents. The GO can be converted to graphite under sever reduction conditions with H2 gas. Fendler, J. H.; Cassagneau, T. Adv. Mater. 1998, 10, 877-881.; Kotov, N. A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. Our method directly employs polyelectrolyte graphite mixtures to assemble LBL electrodes without having to convert the graphite powder to exfoliated GO and then back to graphite where in both processes expensive and sever chemical and thermal conditions applies. The LBL polyelectrolyte-Carbon electrodes [LPCE] achieved most of the requirements stated above, thus providing a cheaper and practical way of making electrochemical electrodes.